Detection of one or more abnormal situations is performed using various statistical measures, such as a mean, a median, a standard deviation, etc. of one or more process parameters or variable measurements made by statistical process monitoring blocks within a plant. This detection may include determination of the health and performance of one or more heat exchangers in the plant, and in particular, detection of a fouling condition of the one or more heat exchangers. Among the statistical measures, the detection may include calculation of an overall thermal resistance of the heat exchanger, which may be indicative under certain circumstances of heat exchanger performance and in particularly degradation of heat exchanger performance as a result of heat exchanger fouling.