A grand canonical Monte Carlo simulation of water condensation in hydrophobic zeolites is reported. A forcefield was developed recently that enabled to reproduce the experimental intrusion-extrusion transition of water in silicalite-1 zeolite at a pressure higher than the saturation vapor pressure. Using this forcefield, we carried out a study of various hydrophobic zeolites (LTA, BETA, FAU, FER) and observed that the condensation pressure could be related to the pore width through a simple Washburn type equation, something that was not expected in such narrow pores. Hysteresis was observed in two systems, while a fully reversible isotherm was found in the other two systems. A mechanism for this interesting feature is provided. The full condensation mechanism of water in hydrophobic nanopores is displayed. It occurs through a clustering effect, followed by a first-order like transition. The final water density was found to be much smaller than the bulk water in the same thermodynamic state point conditions, due to confinment effect. The effect of introducing a hydrophilic defect in the overall hydrophobic framework will finally be described.