This work proposes a novel framework for the optimal design of chemical processes whose main novelty lies in the incorporation of environmental concerns based on the principles of the Life Cycle Assessment (LCA) methodology. The approach presented applies mixed-integer modeling techniques to the superstructure optimization of sustainable chemical process flowsheets. As such, the resulting mathematical formulation simultaneously accounts for the minimization of the environmental impact and the cost. The environmental impact is measured through the Eco-indicator 99, which reflects the advances in the damage-oriented method recently developed for Life Cycle Impact Assessment. The incorporation of this environmental performance measure at the modeling stage causes a multiobjective mixed-integer nonlinear problem (moMINLP) that can be addressed by standard techniques for multiobjective optimization. The main advantages of our approach are highlighted through its application to a well-known design problem (the hydrodealkylation (HDA) of toluene), for which the set of trade-off solutions, in terms of cost and environmental criteria, is computed. The obtained results show that an inherent tradeoff naturally exists between both objectives and also suggest that significant environmental improvements can be achieved if decision makers are willing to compromise,the economic benefit of the process.