A model-based performance monitoring method for a heat exchange process, which is one of the key components of distributed energy supply systems, is developed. This performance monitoring, which utilizes a static input-output model of the process, first estimates the output process variables and heat exchange performance in response to variations in measured input process variables. The estimated output process variables are then compared with the measured ones. In this study, the effectiveness of the performance monitoring method in a transient state with irregular variations in water and steam flow rates is verified through a numerical simulation. The result shows sufficiency of the estimation accuracy of the output process variables and heat exchange performance and the capability of detecting the deterioration in the heat exchange performance in a transient state.