It's not quite that simple because the overall impact of an individual compound relies on both its atmospheric residence time and infrared absorption. This article[1] covers the basics.
That doesn't really contradict adrianN's point in any way. If you integrate methane's heat retention over now to 100 years from now it still beats carbon dioxide. That doesn't mean that if you integrate from 50 years from now until 100 years from now it would still win. Much less so if you look at a geological time period in the future.
What do you think happens to the methane in the atmosphere? It doesn't vanish. Most of it reacts in the statosphere and troposphere and oxidizes into ... you guessed it: Carbon Dioxide.
10% gets eaten by organisms in the soil, but at least 90% turns into CO2.
Of course, Natural Gas thermal combined-cycle electricity generation is MUCH more efficient than Coal, no matter which way you look at it. (Unless you have an old already-paid-for coal plant next to a coal mine and only care about dollars)
