Teaching science with computer simulations is a complex undertaking. This case study examines how an experienced science teacher taught chemistry using computer simulations and the impact of his teaching on his students. Classroom observations over 3 semesters, teacher interviews, and student surveys were collected. The data was analyzed for (1) patterns in teacher-student-computer interactions, and (2) the outcome of these interactions on student learning. Using Technological Pedagogical Content Knowledge (TPCK) as a theoretical framework, analysis of the data indicates that computer simulations were employed in a unique instructional cycle across 11 topics in the science curriculum and that several teacher-developed heuristics were important to guiding the pedagogical approach. The teacher followed a pattern of "generate-evaluate-modify" (GEM) to teach chemistry, and simulation technology (T) was integrated in every stage of GEM (or T-GEM). Analysis of the student survey suggested that engagement with T-GEM enhanced conceptual understanding of chemistry. The author postulates the affordances of computer simulations and suggests T-GEM and its heuristics as an effective and viable pedagogy for teaching science with technology.