This work presents the predictive molecular simulations of a functionalized polymer of intrinsic microporosity (PIM) with an ionic backbone (carboxylate) and extra-framework counter-ions (Na+) for CO2 gas storage and separation applications. The CO2-philic carboxylate-functionalized polymers are predicted to contain similar degrees of free volume to PIM-1, with Brunauer-Emmett-Teller (BET) surface areas from 510 to 890 m(2)/g, depending on concentration of ionic groups from 100% to 17%. As a result of ionic groups enhancing the CO2 enthalpy of adsorption (to 42-50 kJ/mol), the uptake of the proposed polymers at 293 K exceeded 1.7 mmol/g at 10 kPa and 3.3 mmol/g at 100 kPa for the polymers containing 100% and 50% ionic functional groups, respectively. In addition, CO2/CH4 and CO2/N-2 mixed-gas separation performance was evaluated under several industrially relevant conditions, where the IonomIMs are shown to increase both the working capacity and selection performance in certain pressure swing applications (e.g., natural gas separations). These simulations reveal that intrinsically microporous ionomers show great potential as the future of energy-efficient gas-separation polymeric materials.