A novel nonequilibrium molecular dynamics simulation technique has been developed whereby the flux of particles, flowing between explicitly defined regions of different constant chemical potential, is measured directly by counting particles. The method uses both stochastic and dynamic steps. A linear relationship between flux and concentration gradient (Fick's law) is found for methane in a carbonaceous slit micropore with a diffuse wall condition. Diffusion coefficients thereby calculated are larger than transport diffusivities from equilibrium simulations using Darken's rule.