We present a molecular model for the adsorption of CO2, N-2, H-2, and their mixtures in dehydrated zeolite Na-4A. The interatomic potentials for this model were developed by comparing the results of grand canonical Monte Carlo (GCMC) simulations of single-component adsorption at room temperature with experimental measurements. GCMC simulation is also used to assess the adsorption selectivity of CO2/N-2 and CO2/H-2 mixtures, as a function of temperature and gas-phase composition. At room temperature, Na-4A is strongly selective for CO2 over both N-2 and H-2, although this selectivity decreases slightly as the gas-phase pressure increases. Ideal adsorbed solution theory is shown to give accurate predictions of the adsorption selectivity at low CO2 partial pressures, provided that a functional form that accurately describes the CO2 single-component isotherm is used. The adsorption properties Of CO2/N-2 mixtures in Na-4A are compared to the same mixtures in silicalite.