Ionic liquids (ILs) are an exciting class of compounds of high interest from a technological point of view. One of the applications that is raising more interest is their possible use as solvents to carry out the conversion of CO2 into more valuable compounds. Theoretical approaches provide an attractive option to screen ILs properties and give quick answers to guide the experiments, becoming a crucial tool for process design. This work illustrates a practical example based on the solubility of greenhouse and acid gases on the butylmethylimidazolium methylsulfate [C4mim][MeSO4] IL, in order to study its feasibility for gas separation and conversion. A simple but reliable molecular model is presented for the ionic liquid based on structural information and molecular simulations, and coarse-grained models are used to model the different gases. The absorption of relevant gases for the separation/conversion process (CO2, CH4, CO, H2, SO2, H2S) in [C4mim][MeSO4] is modeled and compared with experimental data using a minimum amount of binary data. From this information, the ternary diagrams of [C4mim][MeSO4] with CO2 and the acid gases SO2 and H2S are predicted, and the selectivity of CO2 by respect all the gases is evaluated, with particular attention to the contaminants above mentioned.