The Methane Monster is a one-time villain from the Cartoon Network original series The Powerpuff Girls, only appearing in the episode "Reeking Havoc".
It was voiced by Frank Welker.
Professor Utonium entered Townsville's second annual chili cook-off, with his new kind of chili. In the middle of the night, Professor Utonium was struck with the idea to add a tiny dose of Chemical X to his chili, to give it that extra zing. All he did was add a single drop, so that he did not give it too much mutant chemicals. Unfortunately, after he went back to bed, Blossom was stricken with the same idea to do the same thing, not knowing that Professor Utonium had previously done this to the chili. Blossom added a single drop of Chemical X to the chili, making a total of two drops of Chemical X get put into the chili. After she went to bed, Bubbles had the same idea, and did the same thing. After that, Buttercup did the same thing. All of this Chemical X, being put into the chili did not bode well for it.
The following day, Professor Utonium entered his chili into the contest and the judges loved it. Everything seemed to be going swimmingly, and pretty much everyone in Townsville was clamoring for Professor Utonium's new chili. That night, everyone who had eaten the chili, began having gastro-intestinal problems, and they all started farting. Their stinky farts began filling the air of Townsville, and all the mutated fart clouds started forming together in the sewers, to make one big sentient monster.
When the following morning came, Townsville was under attack by an evil Methane Monster, constructed entirely out of the farts of everyone, who had eaten Professor Utonium's chili. The Methane Monster was torturing everybody with his stink, causing people to pass out, from the scent of his horrendous and pungent aroma.
When The Powerpuff Girls came to the rescue, they were surprised to see that such a creature actually existed. Buttercup dove right into action, thinking she could take him out in seconds, but The Methane Monster barfed out a bunch of stink on her, grossing her out so bad, that she fainted in mid-flight and crashed into a building. Blossom tried landing an attack, but The Methane Monster swung his hand into her person, surrounding her in the stink cloud, causing her to hack and gag. Bubbles didn't bother attacking him, as she wisely noted that none of them would be able to get near the monster, without smelling his disgusting, wretched stench.
Buttercup noticed that the building she crashed into, just so happened to be a nose plug store. She took some nose plugs and divvied them out among herself and her sisters, so that they would be able to get into the battlefield with The Methane Monster, and not be completely repulsed by the stench. The Powerpuff Girls tried attacking him, but being a gaseous cloud, they just went right through him, causing no damage.
Blossom got the idea for them all to blow him away with their super breath, which cause him to deteriorate. This worked in eliminating him for a second, but the tiny deteriorated pieces of The Methane Monster formed tiny versions of itself, which all formed together, to recreate the original character again.
Bubbles had the brilliant idea to suck him in. The Powerpuff Girls used their super breath to inhale The Methane Monster, but this caused them to turn green with disgust and hack and gag him out, being grossed out, even worse than before. Buttercup commented on just how wrong the situation was.
Buttercup admitted defeat, saying that they were no match for The Methane Monster. Blossom got an idea from this. When she heard her say "match", Blossom went to the same place she got a giant jar from, when defeating Roach Coach in the episode "Insect Inside". From there, she retrieved a giant match, which she lit, causing The Methane Monster to vanish, due to the heat of the match, destroying the stench from the farts. The Methane Monster was vanquished to oblivion and Townsville cheered for the heroism of The Powerpuff Girls. The day was saved once again, thanks to The Powerpuff Girls.