A simplified model is applied for the prediction of gas/solid adsorption isotherms of pure gases (i.e., CO2, N2, SO2) on the metal-organic framework Mg-MOF-74, based on periodic Density Functional Theory (DFT) calculations and a dual-site Langmuir approach (DFT/DSL), using a mean-field approximation for the inclusion of the lateral interactions. This model not only provides reliable adsorption isotherms (P ≤ 1 atm, 293 ≤ T ≤ 373 K) but also isosteric heats of adsorption in good agreement with both available experimental data and previous more refined models. Moreover, an extension of this model is used to study the effect of SO2 as an impurity in the efficiency of adsorption and desorption processes of some post-combustion gas mixtures. It is shown that a very low concentration of SO2 is enough to poison Mg-MOF-74 structure. Moreover, different swing adsorption techniques at different working conditions are analysed to reduce the impact of SO2 poisoning in CO2 separation.