## 1 Introduction  
Over the past few decades, energy conservation and air pollutant emission control have been key environmental management issues for the world [1; 2; 3; 4]. As the important source of energy consumption and pollution emissions, the industrial sector has long been the concern for energy conservation and emission reduction, especially for the iron and steel industry [5; 6; 7; 8]. Iron and steel production is accompanied by a large amount of energy consumption and pollutant emissions (including SO\({}_{2}\), NO\({}_{\rm{A}}\), PM, and other pollutants), so how to achieve low-carbon and clean production in the iron and steel industry has always been a concern of the government, enterprises, and scholars. Among many energy conservation and pollutant emission reduction technologies, polygeneration systems have gained attention due to their significant energy, environmental, and economic benefits [9; 10; 11; 12].  
In general, a polygeneration system for the industrial sector consists of two or more industries, and in which waste or by-products produced by some industries can be consumed by other industries [13; 14], thus their overall environmental and energy impact could be reduced, and in some cases, additional economic benefit will also be created. Therefore, polygeneration systems are an important way to promote circular economy and enhance the efficiency of resource utilization.  
There is a large amount of waste or by-products that will be produced in the production process of iron and steel [15; 16], such as steel slag, waste heat, and waste pressure. These wastes or by-products can be used in the chemical, building materials, and power industries. In other words, extensive material and energy utilization pathways exist between the iron and steel, chemical, building materials, and power industries. For instance, coke oven gas and blast furnace gas in the iron and steel industry can be used to manufacture chemical products, blast furnace slag can be used to prepare building materials, and the waste heat and pressure generated from sintering, coking, and blast furnaces can be used to generate electricity.  
The contribution of this study is to establish a comprehensive technical selection framework for the polygeneration system based on the chain of \"supplier--waste/by-products--technologies--adopter\", and the trade-off of energy, environmental, and economic benefits was considered, as well as the trade-off between various environmental targets, such as SO2 reduction and NOx reduction. A multi-objective optimization model was developed in this study to address the above issues, and the iron and steel industry was taken as an example for analysis. The results of this study can provide insights for energy conservation and emission reduction in the iron and steel, power, chemical, and building materials industries, and these methods can also be applied to other industrial sectors.  
The remainder of this study is organized as follows: Section 2 is the literature review. Section 3 introduces the methods and materials, including the polygeneration system simulation, multi-objective optimization model, and data processing. The results and discussion are presented in Section 4, while Section 5 reports the main conclusions of this study and some suggestions for the promotion of polygeneration systems in the iron and steel, chemical, building materials, and power industries in China.