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.