Sector coupling is a central element for enhancing energy efficiency in urban areas. A promising approach is integrating residential energy systems with industrial waste heat recovery through district heating network (DHN). However, it implies the coordination between systems design since a decision taken in one subsystem directly affects the decision-making of other subsystems. This paper aims to illustrate the sector coupling among an energy community, electric vehicle (EV), and an industrial site supplying waste heat. Each building is modeled separately, and the Dantzig-Wolfe decomposition is applied to optimize the district-scale problem. Starting with a global sensitivity analysis, the paper highlights the benefits of coordinated designs across buildings to exploit economies of scales and to enhance PV capacity. Integrating EVs with an uncontrolled charging has minimal impact on cost and emission reductions, whereas load shifting and vehicle-to-grid services increase self-sufficiency by 78%, decreasing total costs by 10% and emissions by 37%. A marginal cost analysis demonstrates how EVs reduce electricity costs in the energy community and lower the marginal cost of heat in the DHN. However, it also reveales that waste heat availability adversely affects the profitability of PV panels in high temperature DHN, leading to competition for local resources. The results emphasise the need for process efficiency and therefore lowering DHN temperature to prevent a spillover effect of industrial inefficiency on renewable penetration.