Crude oil fouling in preheat trains in refineries is usually dominated by organic matter deposition at high temperatures. However, malfunction of desalting equipment, human or technical errors, or changes in feedstock may lead to substantial deposition of inorganic salts or corrosion products, compromising heat exchange performance, pressure drop (hence throughput), and even safety. Understanding how such abnormal deposition and the resulting complex deposit structure affect the thermohydraulic performance of heat exchangers is key to developing adequate monitoring tools for the early detection, diagnosis, and control of the underlying causes. Here, a novel multicomponent fouling deposit formulation is applied to the simulation of deposits composed of organic and inorganic foulants within a single heat exchanger tube. The model enables the tracking of changes and history of local composition in the fouling deposit, thermoconductivity profiles including layering effects, and impact on the overall thermohydraulic performance. The results show that appropriate monitoring of measurable stream conditions, including thermal and hydraulic effects, in combination with reliable predictive fouling and heat exchanger models, allows the detection and (potentially) diagnosing of the abnormal fouling behavior. The model is easily incorporated in full-scale heat exchanger models and is applicable to other processes.