We have performed lattice grand canonical Monte Carlo simulations of benzene adsorption in Na-X zeolite, to determine whether strongly confined benzene molecules exhibit subcritical properties. We observe a phase transition from low to high density of adsorbed benzene, analogous to vapor-liquid equilibrium, at temperatures as high as 300 K and above. By performing thermodynamic integration to construct the coexistence curve, we obtain a critical point for benzene in Na-X at T-c=370+/-20 K, theta(c)=0.45+/-0.05 fractional coverage.