Methane (CH4) is a flammable gas; a chemical compound with a molecular structure containing one carbon atom and four hydrogen atoms.
Titan, a moon of Saturn, has methane lakes on its surface. (TOS - Starfleet Academy novel: The Gemini Agent)
In the year 2365, a sensor scan by the Federation starship USS Enterprise-D registered methane comprising 23% of the atmosphere of the planet Theta VIII. (TNG episode: "The Royale")
Borg drones apparently prefer methane, as it is one of the compounds found in the preferred Borg atmosphere, along with fluorine and carbon monoxide. (TNG movie: First Contact)
In an alternate timeline where the Borg were able to assimilate entire star systems in the Alpha Quadrant, increasing levels of methane were noted in the atmospheres of Alpha Centauri and Andor while the Borg adapted their worlds. (Star Trek novel: Engines of Destiny)