The heat exchanger/reactor(HEX Reactor) focused in this paper is a device with well-characterized performance on both heat exchanging and chemical reaction parts. A detailed nominal model for this HEX Reactor was established before for the use of further researches in the diagnostic section. However, the detailed model is too complex to bring with difficulties in the design of controllers and the amount of calculations. In this paper, a new simplified interconnected model is developed for the HEX Reactor. The new methodology tries to make full use of the sensors. The interconnected cells in the model are constructed in different sizes according to the locations of the thermocouples. In this way, the total amount of subsystems is evidentially decreased and the nominal model is therefore simplified. A series of experiments in the heat exchange part have been carried out. The experiment data is then compared with that of the simulations on the simplified model. Results show a very high precision of the new model to the physic HEX Reactor.