For a cathode duct of a proton-exchange membrane fuel cell, a three-dimensional analysis method is further developed to include two-phase, multicomponent gas and heat transport processes. A set of momentum, heat transport, and gas species equations is solved for the whole duct by coupled source terms and variable thermophysical properties. The effects of the electrochemical reactions on the heat generation and mass consumption/generation are taken into account. The effects of liquid water on the local current density and cell performance are discussed by incorporating the Tafel formula and a liquid-phase saturation function. The numerical predictions are compared with experimental results, and good agreement is demonstrated between the present results and the measured ones in terms of polarization curve.