The net mass transfer resistance for gas molecules permeating through zeolite membranes includes contributions from intracrystalline diffusion and contributions from interfacial effects. These interfacial effects can arise either from gas-zeolite interfaces or from interfaces that exist within zeolite crystals due to grain boundaries. We present the first atomically detailed simulations that examine interfacial mass transfer resistance due to internal grain boundaries in zeolites that are relevant for membrane applications. Our calculations examine twinned silicalite crystals in crystallographic configurations that have been identified in previous experiments. We used the dual control volume grand canonical molecular dynamics method to simulate the permeance of CH(4) and CF(4) through thin twinned silicalite crystals. The magnitudes of the grain boundary resistances are quite substantial, at least for the thin crystals that are accessible in our simulations.