In the heat-exchanger network (HEN) literature, synthesis, design, and flexibility analyses of HENs are done independently from processes to which HENs are integrated. Such analyses are made mostly based on nominal operating conditions at which the HEN's source- and target-stream properties are evaluated. However, terminal-stream properties of HENs depend upon temperatures, pressures, and compositions of the process connected to the HEN. In this work, flexibility and operability issues of a HEN are investigated with rigorous simulations using the process flowsheet simulator HYSYS for a HEN-integrated natural gas turbo-expander plant (TEP) operating under ethane-recovery mode. The contribution of this work is threefold. First, the HEN-plant interactions are exemplified via the process flowsheet simulator. Second, flexibility and operability issues are tackled using the optimization capability of the flowsheet simulator. Third, for highly energy-integrated complex plants like the TEP, the difficulties or impossibilities of automated HEN synthesis and flexibility analysis with process flowsheet simulators are demonstrated. (c) 2005 Elsevier Ltd. All rights reserved.