Grand canonical Monte Carlo (GCMC) simulations have been performed to investigate the adsorption and separation behavior of ternary and quaternary gaseous mixtures of CO2, along with H2S, SO2, and N2, in bundles of aligned double-walled carbon nanotubes with a diameter of 3 nm and an intertube distance of 0.5 nm. All of the simulations are performed at 303 K and at pressures varying between 0 and 3 bar. The GCMC results are then compared to the ideal adsorbed solution theory (IAST) predictions. For the ternary mixture H2S–CO2–N2, the results show that CO2has the highest adsorption among the three components. The IAST predictions agree reasonably well with the GCMC data for the ternary mixture, except for H2S. For the quaternary mixture H2S–SO2–CO2–N2, it is observed that initially CO2has the highest adsorption up until around 2 bar, whereafter there is a crossover by SO2to have the highest adsorption. IAST fails to predict the adsorption behavior of the quaternary mixture involving SO2.