Difficult to say--I'm no atmospheric physicist, but I'm guessing that insolation and albedo will factor heavily in the final equation, as will the transparency of the atmosphere.

If the insolation were similar to Earth, and the percentage of methane were say 5-10%, I'm guessing the temperature would be *very* hot, especially if there's also water vapor; methane is a much more potent greenhouse gas than carbon dioxide.

__________________
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--The State