This greatly depends on the design and mechanical characteristics of combustion (exhaust temperature, etc.) and steam turbines (reheat/non-reheat), and, especially, HRSGs (duct firing, single-or multi pressure, economizer, etc.).
It also depends on the type of fuel.
For example, CC plants based on H-series GE GTs can achieve up to 60% thermal efficiency (LHV) burning natural gas. Integrated Gasification CC plants (burning syngas), built on the same units, are at around 45-49%.

Alexei