Large dual-purpose power desalination plants are used to reduce the production cost of both electricity and water. Various combinations of power-desalination systems are conceivable in order to satisfy both power and water demands. The preference of one scheme over another would depend mainly on many factors, such as the required power to water ratio, cost of fuel energy charged to the desalting process, electricity sales, capital costs, and local requirements. The allocation of the total annual cost of a dual-purpose plant to desalted water and electricity can be made by various methods. In this paper, credit method is adopted to calculate the total annual cost. Credit methods allocate a predetermined value to one of the products and determine the cost of the other product by subtraction from the total cost of the dual-purpose plant. In this way, according with the power credit method, the total cost is calculated in the following way: C-water =C-total -W, where W is the benefit of the net electrical generated. Non-linear equations are extensively used for the cost equipment and for the plant performance. The aim of this paper is to design a dual-purpose plant system at minimum cost determining the equipment configuration and its corresponding operating conditions given the water production. The optimal solution is selected from a superstructure containing different possible configurations. One case study illustrating the methodology, robustness and computational performance is presented.