In this work, our previous simulation method on the calculation of solubility of nonpolar solutes in nonpolar polymers [H. Eslami and F. Müller-Plathe, Macromolecules 40, 6413 (2007)] has been extended to the case of solubility calculation for water, as a polar penetrant, in poly(ethylene terephthalate), as a polar polymer. The chemical potentials of water in the polymer phase and in the gas phase have been calculated by employing our grand canonical ensemble molecular dynamics simulation method [H. Eslami and F. Müller-Plathe, J. Comput. Chem. 28, 1763 (2007)]. In this paper it is shown that performing just two independent simulations, one in the polymer phase and one in the vapor phase, in the grand canonical ensemble, is sufficient to calculate the phase coexistence point. The calculated solubilities, diffusion coefficients, and permeability coefficients are in good agreement with experimental data. Also the calculated glass transition temperature of the wet polymer is shown to be in a very good agreement with experiment.