Abstract: The invention relates to a method for evaluating fouling of the passages of a spacer plate (10) of a shell-and-tube heat exchanger (11), wherein said passages (12a, 12b) are provided along the tubes (11) to allow fluid to flow through the spacer plate (10), wherein, for each of a plurality of at least one passage (12a, 12b): at least one measurement of a parameter dependent on fouling or the presence of magnetite is made in the vicinity of the passage, using an eddy current probe; this measurement is used to derive at least one indicator of the fouling of said passage, characterized in that said fouling is evaluated by comparing a set of one or more fouling indicator vectors of dimension of at least two, built from the fouling indicators thus obtained with a plurality of fouling indicator vector sets contained in a database, wherein each of said indicator vector sets is associated with a quantitative descriptor of fouling.