An integrated biorefinery is a processing facility that converts biomass feedstocks into a wide range of value added products which include biofuels and specialty chemicals. In order to synthesize a sustainable integrated biorefinery, the environmental and economic performances should be taken into consideration. Via heat and power integration within integrated biorefinery, both performances can be improved. Due to the complexity of the process synthesis and integration problem, it is a need for a systematic approach to address the problem. In this work, modular optimization approach which breaks a large optimization problem into small models is proposed. Each model consists of a process unit and its alternatives (i.e., biomass drying, gasification, synthesis processes, etc.) in different degree of modeling details (i.e., white-, grey- or black-box). This allows designer to 'zoom in' on specific key process units, while, the rest of the model remain as black boxes. Thus, it will be able to reduce the computational effort significantly as compared to pure white-box modeling. By using an effective objective function to maximize the economic potential of the overall preliminary conceptual design, selection of the optimum process alternatives/technologies and products as well as integration of heat and power between process units can be performed simultaneously. A case study is solved to illustrate the approach.