For efficient natural gas upgrading at near ambient temperatures, moisture-pretreated lithium is proposed as a reactive adsorbent, and its separation performance was examined for selected gas mixtures (involving combination of CH4, CO2, O-2, N-2, and/or H-2) by theoretical studies. For evaluating the proposed adsorbent, we used density functional theory calculations as well as temperature-programmed kinetic Monte Carlo calculations. Contributions of different mechanisms for the gas capture were examined, including diffusion, adsorption, and reaction. Among these mechanisms, reaction was found to be the dominant factor during the gas-capture process. In contrast to the conventional reactive adsorbents where gases directly react with adsorbents and thus are captured, the gas-capture mechanism herein follows the reaction between gases and species generated upon pre-exposure of lithium to moisture. The proposed separation process is thermodynamically favorable and would progress spontaneously. It is shown that for several gas mixtures containing methane, a perfect selectivity can be achieved in which a high-purity methane gas stream is the final gas product while the other gaseous components are transformed to corresponding solid products.