The aim of this paper is to present the development of a simulation tool in order to assess the inherently safe characteristics of a heat-exchanger reactor (HEX) operating reaction systems. The modelling of steady and transient states of a HEX reactor is performed following a hybrid dynamic approach. The global dynamic behaviour of this reactor can be represented by several continuous models, which are bounded by state or time events. Each continuous model is defined as a system of partial differential-algebraic equations. The numerical scheme is based on the method of lines. Special attention is paid to the model initialization and a simulation strategy of the start-up phase is presented. The validation of the model is made by numerous examples, such as the simulation of an exothermic reaction.