When the polymers are studied by interaction chromatography (IC) in porous media, the IC separation mechanism competes with the size-exclusion chromatography (SEC) mechanism and under specific conditions close to the critical adsorption point (CAP), the elution times of monodisperse polymer samples nonmonotonically depend on pore sizes. We performed Monte Carlo (MC) simulations to elucidate this intriguing effect. By analyzing the behavior of self-avoiding and intersecting chains in two-dimensionally (2D)-confining square pores and in 1D-confining slits in good and Θ-solvents, we confirmed that the dimensionality of the confinement, more specifically, pore geometry, controls the chromatographic behavior. The nonmonotonic dependence of chromatographic characteristics on pore sizes occurs only in separations of self-avoiding chains on stationary phases composed of 2D-confining pores with strongly interacting walls. In agreement with experimental observations, the partition coefficient, K, increases with pore size, D, in narrow pores, peaks and then decreases in wider pores. The combination of thermodynamic and conformational analyses clearly showed that a complex interplay between enthalpy and entropy in 2D-confined media explains the nonmonotonic pore size dependence observed in the IC regime. The study specifies the region of conditions which endanger unambiguous interpretation of elution curves. Because the interplay of steric and adsorption effects takes place not only in chromatography, but also in other separation techniques (e.g., gel electrophoresis, nanofluidic techniques), the conclusions are generally relevant for all separations of large molecules in porous media.