Our study of fixed feature importance solidifies many existing membrane design principles, but additionally offers unique, generalized guidance for the molecular engineering of new polymers for gas separations. Overall, SHAP analysis illuminates the chemical balancing act required for overcoming the permeability/selectivity tradeoff. Polymers must juggle (A) the number of bulky chemical moieties-i.e. methyl groups, aliphatic rings-that increase microporosity (permeability at the expense of selectivity) with (B) the number of polar groups-i.e. carbonyls, oxygens-that increase relative CO2 and O2 affinity (selectivity at the expense of permeability).