Liquid-vapor equilibrium, criticality, and spinodal transitions in nanopores are studied by the gauge cell Monte Carlo simulation method proposed recently (Neimark, A.V.; Vishnyakov, A. Phys. Rev. E 2000, 62, 4611). As an instructive example, we consider the capillary, condensation of argon in cylindrical pores of different diameters (1.5-5.5 nm) representing typical pore channels in mesoporous molecular sieves. At the subcritical conditions, the gauge cell method allows one to construct continuous phase diagrams in the form of a van der Waals-type sigmoid isotherm. The sigmoid isotherm contains stable and metastable states on the adsorption and desorption branches connected by a backward trajectory of thermodynamically unstable states which cannot be observed experimentally yet can be stabilized in simulations. The phase equilibrium is determined by thermodynamic integration along the sigmoid trajectory using the Maxwell rule. The spinodals give the true limits of stability of vaporlike and liquidlike states. A notable difference was found between the spinodals and the limits of stability of the vaporlike and liquidlike states achieved in grand canonical Monte Carlo simulations. The critical conditions of the first-order vapor-liquid transition in pores were determined. Good agreement with experimental data on argon adsorption at 87 K on mesoporous molecular sieves was found for equilibrium transitions in pores wider than 2.2 nm and for hysteretic adsorption-desorption isotherms in pores wider than 5 nm.