A numerical method of analyzing closed system feedwater heaters is presented. The numerical results are validated using data generated from the classical log mean temperature difference (LMTD) method. It is found that the numerical results are very close (±1%) to the ones predicted by the LMTD method. Temperature and heat transfer distributions along the length of the heat exchanger are presented. A general approach to determine area allocations among the desuperheating, condensing and subcooling zones under a known set of operating conditions is presented for a feedwater heater in a steam power plant. It is shown through an example problem that a significant amount of heat duty (about 80%) is handled by the condensing zone, whereas the subcooling zone handles the least amount of heat duty, which essentially vanishes at low steam pressures, for example, at a pressure less than 400 psi.