Molecular simulations were used to investigate separation of CO2 from CH4 and N-2 in a recently synthesized microporous metal-imidazolate framework (MMIF). Single component adsorption isotherms of CO2, CH4, and N-2 in MMIF were computed using Grand Canonical Monte Carlo simulations, and a good agreement between simulations and experiments was found. Binary mixture adsorption isotherms were also computed and the validity of the ideal adsorbed solution theory was tested. Effects of bulk gas composition, temperature, pressure, and electrostatic interactions on the adsorption selectivity were investigated. MMIF outperformed many other metal organic frameworks and zeolites in adsorption-based CO2 separations. More interestingly, molecular dynamics simulations showed that diffusion of CO2 is several orders of magnitude larger than the diffusivity of CH4 in the pores of MMIF. This makes MMIF a very promising material for kinetic separations with an unprecedentedly high CO2/CH4 selectivity.