As boiler plants which supply steam and heat into chemical plants become larger in scale and more complicated, accidents caused by the wrong understanding and erroneous operations are increasing. In this paper, we present a computer-aided system that should prevent such mistakes and synthesize operating procedures for start-up of steam boiler plants: 1) This system generates a goal tree by generalizing and classifying the operational situations hierarchically, and uses the relationship between an operation and a function to search for primitive operations in the lowest level of the goal tree. 2) It simulates changes in operational situations caused by the action of primitive operations with heuristic knowledge of transition relationships between operational situations. 3) It determines the priority of low-level operational situations and primitive operations by referring to the database on hazardous situations and properties. We have applied the developed system to a large-scale boiler plant to demonstrate its effectiveness.