(covers information from several alternate timelines)
Methane is an odorless, colorless simple hydrocarbon with the chemical formula CH4. It is the major component of natural gas that can be combusted to produce energy. It exists on many planets, including Earth.
In 2151, the Klingon courier Klaang led two Suliban pursuers into a grain silo on Earth, owned by a farmer named Moore. The silo contained methane, which Klaang detonated behind him, killing the Suliban. (ENT: "Broken Bow")
In 2370, the USS Enterprise-D inspected the vessel USS Raman via an interface probe controlled by Geordi La Forge. The probe entered a hull breach in the aft section of the Raman and detected atmospheric gases inside the corridor – primarily methane and ammonia. (TNG: "Interface")
In an alternate timeline where the Borg assimilated Earth after stopping Zefram Cochrane's warp flight, the planet had an atmosphere of methane, carbon monoxide, and fluorine in 2373. (Star Trek: First Contact)