Ideal Adsorbed Solution Theory (IAST) is a predictive model that does not require any mixture data. In gas purification and separation processes, IAST is used to predict multicomponent adsorption equilibrium and selectivity based solely on experimental single-component adsorption isotherms. In this work, the mixed gas adsorption isotherms were predicted using IAST calculations with the Python package (pyIAST). The experimental CO2 and CH4 single-component adsorption isotherms of Mg-gallate were first fitted to isotherm models in which the experimental data best fit the Langmuir model. The presence of CH4 in the gas mixture contributed to a lower predicted amount of adsorbed CO2 due to the competitive adsorption among the different components. Nevertheless, CO2 adsorption was more favorable and resulted in a higher predicted adsorbed amount than CH4. Mg-gallate showed a stronger affinity for CO2 molecules and hence contributed to a higher CO2 adsorption capacity even with the coexistence of a CO2/CH4 mixture. Very high IAST selectivity values for CO2/CH4 were obtained which increased as the gas phase mole fraction of CO2 approached unity. Therefore, IAST calculations suggest that Mg-gallate can act as a potential adsorbent for the separation of CO2/CH4 mixed gas.