High-temperature exhaust streams contain substantial recoverable energy, but simple organic Rankine cycle (ORC) performance is often governed by interacting thermal and fluid-property constraints. This study develops a physically constrained optimization, exergy assessment, and sensitivity framework for a simple subcritical ORC using low-global-warming-potential R1233zd(E). A fixed-composition natural-gas-turbine exhaust enters at 543 °C and 80.4 kg s⁻¹, with temperature-dependent properties evaluated using NASA7 ideal-gas correlations. The counter-flow heat-recovery vapor generator includes a 15 K pinch constraint, a 120 °C minimum stack temperature, and cooling-water approach limits. Decision variables are evaporation-pressure ratio, condensation temperature, and superheating degree; working-fluid properties are calculated with CoolProp 7.2.0. An independent numerical rerun reproduces the same constraint-controlled boundary design within numerical tolerance. The retained design delivers 6119.49 kW net power and 16.388% thermal efficiency at an evaporation-pressure ratio of 0.849996, condensation temperature of 38.000 °C, and 4.580 K superheating. The stack-temperature, condenser-approach, and critical-temperature safety constraints are active, while the actual minimum HRVG temperature difference is 80.18 K, confirming that the pinch constraint is inactive. The hot-gas physical-exergy decrease is about 18.22 MW, with a net physical-exergy recovery ratio of 33.58%. Exergy analysis identifies the HRVG as the main source of irreversibility. Overall, the results show that source–working-fluid temperature mismatch limits the suitability of a simple subcritical R1233zd(E) cycle for high-temperature exhaust recovery.