The best replacement for methane is co2.

The best alternatives to methane are rated as following:
★★★★★  co2
★★★★  natural gas
★★★★  carbon dioxide
★★★  methanol
★★  ethane
★  butane
★  methyl
★  c02
  methane
  pentane
  octane
  ch4
  biogas
  acetylene
  co2 global warming
  ethane have a higher boiling point
  ammonia more soluble in water

Your alternative for methane is not listed? Suggest!
Speaking about methane, one probably shouldn't forget about bromine.

Other search terms related to methane and their ratings are:
★★★★★ bromine
★★★★ chlorine
★★ oxygen reaction
★★ bromine water
★ fluorine
You have in mind a companion for methane and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of methane in all years. Click on graph for more.
methane
co2
natural gas