NH₃/H₂O based systems are promising for thermal energy storage and thermal energy conversion. These systems are used for absorption energy storage and Kalina cycles. This paper investigates the condensation of high-concentration NH₃/H₂O in vertically downward plate heat exchangers. A combined method is proposed by discussing the applicability of equilibrium and non-equilibrium models. Both models are necessary for zeotropic mixtures with large temperature glide. The non-equilibrium model applies where the temperature glide is non-linear or the vapor is in non-equilibrium with the liquid. The equilibrium model becomes applicable with decreasing vapor qualities. Heat transfer correlations are proposed according to the equilibrium model, which interpret convective condensation and gravity-controlled condensation. The additional heat transfer resistance is calculated considering mass transfer. The non-equilibrium model is further developed quantifying the heat and mass transfer of vapor and neglecting the mass transfer resistance of the liquid. The non-equilibrium model transforms into the equilibrium model as the concentration gradient of vapor approaches zero. Additionally, a frictional pressure drop model for separated flow conditions is proposed and quantifies the two-phase shear force.