An intelligent inversion algorithm has been developed to determine the path averaged concentration of methane over wetlands from open-path Fourier-Transform InfraRed (FTIR) measurements. The algorithm, based on a Doesn't-Use-Derivative (DUD) non-linear least-squares method, is robust and yields accurate methane concentrations for all atmospheric conditions. Fifty-eight spectral channels were needed to accurately determine the concentration of methane in the presence of other absorbing gases, especially for water vapor at 100 percent humidity. Tests were made for mixtures of the first 12 gases on the HITRAN database at typical ambient concentrations. The algorithm is described with particular attention to applications of methane fluxes over Northern-Hemisphere wetlands.