Carbon nanotubes have significant promise as gas separation membranes. Gas permeation through nanopores involves mass transfer resistances from molecules entering and leaving pores (so-called surface resistances) and diffusion within the pores. We use molecular simulations to give the first estimates of surface resistances for gas transport through nanotubes. For CH4 transport through ( 20,0) carbon nanotubes at 300 K, surface resistances are small for nanotubes 5-10 mu m in length but can be significant for shorter nanotubes.