We present and analyze isothermal adsorption and desorption measurements for methane physisorbed on a Cu(100) surface to characterize the interplay between molecule-surface and molecule-molecule interactions. A 2D gas-liquid phase boundary for the methane layer is observed and partially mapped in coverage and temperature. Comparing our measured critical temperature of 66 K +/-2 K to previous work on related systems implies an attractive well between physisorbed molecules of similar to 130 K. This reduction compared to the gas phase well can be adequately understood in terms of dielectric screening of the dispersion interaction, By contrast, analyzing the low-coverage adsorption and desorption isotherms using a quasiequilibrium model and imperfect gas statistics, we deduce a lateral attractive intermolecular potential well of only 30-50 K. This drastic reduction of the attractive well from the gas phase value is not consistent with image charge screening, and is difficult to reconcile with the observed critical temperature. This discrepancy is discussed in terms of surface corrugation which dictates different positions for molecule-surface and molecule-molecule potential energy minima.