In this study, the effect of pore textural property and surface functionalization in activated carbon (AC) on the competitive adsorption and diffusion of a binary mixture of CO2/CH4 was examined by grand canonical Monte Carlo simulations and equilibrium molecular dynamics. The simulation results indicated that AC with a high surface area exhibited a low CO2/CH4 selectivity, while a high CO2 adsorption capacity was observed. To obtain high CO2 adsorption capacity and high CO2/CH4 selectivity at the same time, surface chemistry (adsorbent adsorbate interaction) and pore textural property (pore size) were investigated. On this basis, two types of AC were prepared: (I) with different pore sizes, attributed to building ordered pores and (II) with different surface chemistry properties, attributed to modification with different functional groups. The results showed that the effects of surface chemistry properties are factors more important for improving the CO2/CH4 capacity and selectivity.