Computer simulations of xenon gas diffusion within zeolite NaX have been performed utilizing the hard-sphere interaction. By employing the hard-sphere interaction, it is possible to perform a simulation for 20,000 to 40,000 xenon gas atoms diffusing into a microscopically sized zeolite NaX crystal. Also, a desorption simulation is performed following each absorption. The Fickian diffusivity coefficient, as well as the Einstein diffusivity coefficient, is obtained in each simulation and the observed value closely matches that obtained experimentally. Most importantly in any simulation, the evaluated desorption coefficient is consistently less than that for absorption. This result is obvious from observing the temperature profile of xenon gas atoms during absorption and desorption. These simulations are performed using a PC and are currently in progress for xenon gas at different temperatures and molar densities.