In the energy transition context, the design of integrated multi-energy systems is key for reaching ambitious sustainability objectives. Due to the intermittent nature of the renewable energy sources, introducing technologies for storing and transforming energy in different carriers (e.g., electricity, gas, heat) is, in fact, a strategic solution for fully exploiting the renewable power generation, increasing the flexibility of the system, and contributing to the decarbonization. Although the need to rely on multi-energy systems is widely shared, identifying their optimal design requires the use of complex modelling tools able to characterize the territory, simulate the system dynamics, and evaluate the solutions with respect to different sustainability objectives. To support the decarbonization decision-making process, in this work we develop a three-step modelling chain for planning optimal multi-energy systems at the local scale. More precisely, we first perform a territory characterization by estimating, through different methodologies, input data of renewable resource availability, territory exploitation potential, and energy demand of electricity and heat. Then, we carry out a multi-energy analysis identifying Pareto optimal system designs with respect to two sustainability objectives, namely the Net Present Cost and the CO₂ emissions. Finally, we perform an intersectoral Multi Criteria Analysis-Cost Benefit Analysis (MCA-CBA) for evaluating the solutions obtained in the previous step with respect to a wide range of indicators representing energy, economic, and social acceptance aspects. The CBA approach is adopted for evaluating the financial and economic viability of the investment options, while the assessment of non-monetary impacts is performed through the MCA approach. We apply the modelling chain to the real case study of Sulcis Iglesiente (Sardinia, Italy), a territory characterized by carbon-intensive industries, recently selected for receiving funding from the Just Transition Fund launched by the EU Commission in the context of the Green Deal. Expected results aim to demonstrate the validity of the proposed modelling chain in the identification of the best interventions for supporting the decarbonization and the sustainable development of Sulcis Iglesiente.