The numerical simulation of nonequilibrium cotransport of H2 and alkane gas mixtures with very different molecular sizes through a porous carbon membrane structure was implemented. Simulated permeabilities and selectivities in binary diffusive systems (and in one ternary system), at pressure about tens of atmospheres and at operating condition of room temperature or higher, can be predicted from single-gas permeabilities. This suggests that the effect is geometrical and an approximate model of the transport is proposed. It can be used for an estimation of the separation factor of a membrane. Simulations are compared with experimental results of two- and three-component codiffusion and counterdiffusion in a carbon membrane. It is shown that diffusion in a porous molecular network and in a carbon nanotube are completely different. The uniqueness of this work lies in the comparison of simulated, approximate, and experimental results, which enables us to identify the important parameters.