A mathematical optimization approach for targeting the minimum utility consumption in heat-integrated process water networks is proposed in this paper. The linear programming formulations are developed for heat integration in fixed flow rate water allocation networks, for both single and multiple contaminants, incorporating isothermal mixing. Heat integration in water allocation networks is also addressed through nonisothermal mixing of streams, and this is formulated as a discontinuous nonlinear programming problem. Utility requirements, for isothermal as well as for nonisothermal mixing, are compared over a range of minimum approach temperatures to evaluate the energy performance using illustrative examples. The number of required heat exchangers is less in heat-integrated water allocation problems with nonisothermal mixing. Simultaneous optimization of the overall heat-integrated water allocation network, to minimize the operating cost, is also formulated and solved.