Extensive research for BDI (Belief, Desire, Intention) agents has been carried out. To employ BDI agents to an application such as Life Cycle Costs (LCC) information gathering, a methodology is required to analyze, design and specify agents from system requirements. Although some methodologies are available for development of agent-based systems, none of these provide a systematic approach for BDI agent-based systems involving all the phases of software development life cycle, especially for addressing agent specifications. This paper exploits synergy across different research areas such as use cases, BDI agents, role models, and IDEF, and proposes a methodology for BDI agent-based systems. This methodology presents a step-by-step process for agent-oriented software engineering. This process has been applied to designing a BDI agent-based information gathering system for LCC.