Heat exchangers in which phase changes occur are widely applied in industry. Condensing vapor streams and boiling liquid streams are important components in almost all flowsheets of chemical processes. The accurate modeling of these systems is important at the design stage so that heat exchangers with adequate heat-transfer area are purchased and installed. The purpose of this paper is to illustrate that some caution must be exercised in developing simulation models for heat exchangers when liquid is vaporized or vapor is condensed. Use of simple standard heat exchanger models can lead to incorrect differential temperature driving forces and gross underestimation of heat-transfer area. (C) 2014 Elsevier Ltd. All rights reserved.