## 5 Conclusions  
In this study, the iron and steel industry is selected as the by-product industry, while the chemical, building materials, and power industries are selected as the sink industries to simulate the polygeneration system, and four typical polygeneration paths are set up. And then, the technology structure system of the polygeneration process is modeled based on the bottom-up method, and the modeling process follows the chain of \"supplier--waste/by-products--technologies--adopter\". Based on the above, a polygeneration process model including eight types of by-products and 20 technologies has been established. Apart from this, a multi-objective optimization model is developed in this study to select the optimal technology or technology combination of each polygeneration path under the constraints of energy, environmental, and economic objectives, as well as guide enterprises to adopt the optimal path to realize the comprehensive utilization of by-products. The main conclusions of this study are concluded below. 1. The energy conservation and emission reduction measures have an impact on multiple goals such as energy, environment, and economy during the application process. There are complex collaborative and conflicting relationships between different targets, and there is a risk of implicit transfer between various goals. The multi-objective optimization algorithm represented by the NSGA-II algorithm can well handle the above issues, which can provide various solutions for decision-makers, and decision-makers can formulate energy conservation and emission reduction strategies based on their preferences. 2. There are synergistic and conflicting relationships among energy, environmental, and economic goals in the process of applying polygeneration technologies in the iron and steel industry, which is also closely related to the scale of iron and steel plants. Therefore, it is necessary to make technology selection to avoid conflicts between different objectives when establishing the polygeneration system, as well as maximize the energy, environmental, and economic benefits for iron and steel plants with different scales. 3. Polygeneration systems can significantly improve the energy conservation and pollutant emission reduction potential of the iron and steel industry, which is crucial for achieving the pollution and CO\({}_{2}\) reduction targets of China's industrial sector in the future. Polygeneration technologies can fully utilize waste or by-products in the iron and steel plants, such as coke oven gas, sintering waste heat, and steel slag, which will be converted into industrial products, including chemical products, electricity, building materials, and others. The independent energy conservation and emission reduction potential of a single industry is limited currently. And it has become a trend for the industrial sector to fully utilize by-products/wastes from different industries through the establishment of polygeneration systems to save energy and reduce emissions. In the future, it is necessary to focus on the multi-level utilization of energy in the industrial sector and the application of circular economy measures. In conclusion, critical insights are provided in this study in the energy and environmental management of iron and steel through establishing an inter-industry polygeneration system with the iron and steel industry as the core and multi-objective optimization model. The results of this study can provide references for energy conservation and emission reduction management in the global iron and steel, chemical, building materials, and power industries. Conceptualization, M.X., Y.W. and Q.L. ; methodology, M.X. and Y.W. ; formal analysis, M.X. and Y.W. ; investigation, M.X., S.L., Y.W. and Q.L. ; data curation: M.X. and Y.W. ; writing--original draft preparation, M.X. ; writing--review and editing, M.X., S.L., Y.W. and Q.L. ; funding acquisition, M.X. and Q.L. ; project administration, M.X. and Q.L. All authors have read and agreed to the published version of the manuscript.