This review examines research on marine lean burn spark ignition (SI) engines powered with natural gas (NG), focusing on a 500 kWe engine with a flat cylinder head and hemispherical bowl-in piston. These engines demonstrate cylinder flow regimes that significantly affect combustion and heat transfer. The current heat-transfer models can’t capture and assess how cylinder flow regimes influence combustion and heat transfer, as they’re validated on conventional engine testbeds. This creates a gap in accurately simulating heat transfer for marine SI engines, underscoring the need for targeted model assessment. Here, these heat-transfer models are applied, compared, and evaluated under full-scale, multi-cylinder, lean-burn conditions to assess modeling accuracy and sensitivity to key parameters. By identifying which correlation best predicts in-cylinder heat loss, this review lays the groundwork for more reliable thermal modeling and optimized natural-gas spark-ignition (NG-SI) retrofit strategies.