Metal Tube wall temperature in smoke tube boiler

[bytejuice2]

HELP
I have a Waste Heat Boiler type smoke tube.
In tube side I have gas from Reaction Furnace with H2S at 1322°C inlet and 280°C outluet.
shell side is water at 140° inlet that produces steam at 14.8 bar - 200°C.
Tubesheets have refractory to prevent heat damages.
Inlet side of Tubes have a ceramic sleeves 250mm long.
Design temperature of tube side is 340°C.
The tubes are ALWAYS under water to avoid superheating.
how can I calculate Metal Mean wall temperature of tubes?
thank a lot
bytejuice2

 

[bytejuice2]

I forgot materials.
shell/tube side is A.516.70 / A.106 Gr.B / a.105
Tubes are Astm A.106 Gr.B.