Nowadays, increasing awareness of sustainability and varied customer requirements have driven manufacturers to reconsider product development starting from the design phase. As one response to these concerns, modular product design (MPD) has attracted significant attention. Since the architecture of a product or a system can have implications on its assembly cost, MPD and product assembly should be investigated jointly. In this paper, a heuristic clustering algorithm with key components emphasis that will reduce assembly cost is offered to group components into modules. Key product/system components are those that afford competitiveness to a company. An MPD method that can decrease product assembly cost while accommodating key components strategically is the primary motivation for this research. Upon the foundation of extant works, we provide the details of the proposed methodology and illustrate its use via a coffee maker case study.