The CO2 solubility data in the ionic liquid (IL) 1-allyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide, methanol (MeOH), and their mixture with different combinations at temperatures of 313.2, 333.2, and 353.2 K and pressures up to 6.50 MPa were measured experimentally. New group binary interaction parameters of the predictive universal quasichemical functional-group activity coefficient (UNIFAC)-Lei model, which has been continually advanced by our group, were introduced by correlating the experimental data of this work and the literature. The consistency between experimental data and predicted results proves the reliability of UNIFAC-Lei model for CO2-IL-organic solvent systems. The newly obtained parameters were incorporated into the UNIFAC property model of Aspen Plus software to optimize a conceptual process developed for the purification of a CO2-containing gas stream. The simulation results indicate that the use of IL either mixed with MeOH or purely considerably lowers the process power consumption and improves the process performance in terms of CO2 capture rate and solvent loss. (c) 2018 American Institute of Chemical Engineers AIChE J, 64: 2168-2180, 2018