In petrochemical process plants, there are many heat exchangers using industrial water as coolant. Further, water treatment systems usually recycle the water in order to minimize the environmental loads. Optimization of maintenance management of the heat exchangers and treatment of industrial water is needed since the quality of industrial water strongly affects the corrosion rate of tubes in heat exchangers and the decrease of their heat transfer coefficients. In this paper, a method is proposed to obtain the optimum conditions utilizing multivariate analysis based on data collected in the daily works of maintenance and water treatment. A concrete example of applying this method to 31 existing plants and optimizing PDCA management of maintenance by improving the sum of maintenance cost and water treatment cost is shown.