Recycle/reuse networks are commonly used in industrial facilities to conserve natural resources, reduce environmental impact, and improve process economics. The design of these networks is a challenging task because of the numerous possibilities of assigning stream (process sources) to units that may potentially employ them (process sinks). Additionally, several fresh streams with different qualities and costs may be used to supplement the recycle of process streams. The selection of the type and flow of these fresh resources is an important step in the design of the recycle/reuse networks. This work introduces systematic approaches to address two new categories in the design of recycle/reuse networks: (a) The incorporation of thermal effects in the network. Two new aspects are introduced: heat of mixing of process sources and temperature constraints imposed on the feed to the process sinks iv (b) Dealing with variation in process sources. Two types of source variability are addressed: flowrate and composition For networks with thermal effects, an assignment optimization formulation is developed. Depending on the functional form of the heat of mixing, the formulation may be a linear or a nonlinear program. The solution of this program provides optimum flowrates of the fresh streams as well as the segregation, mixing, and allocation of the process sources to sinks. For networks with variable sources, a computer code is developed to solve the problem. It is based on discretizing the search space and using the concept of "floating pinch" to insure solution feasibility and optimal targets. Case studies are solved to illustrate the applicability of the new approaches.