I have to control the outlet temperature of natural gas process fluid, which flows in the tube side of a vertical beu HEX.
At the shell side steam is condensates. By modulate steam mass flow with a throttling control valve you can control natural gas temperature. Classic temperature control method at HEX.
The problem is that in any case condensation pressure is arises from the heat and mass balance into the HEX. At very low load pressure will decreased and it may reach even 1barg. Then it is difficult for condensate to be transmitted in to the atmospheric flash tank.
In cases that a condenser shall operates at a variable loads with a significant range then HEX pressure will varies a lot. You need then a pressure control system.
The key is to install (and not the only solution) a control valve at condensation line in order to create a level at low loads and reducing thus saturation area. Then HEX pressure will increased.
The question is can you achieve with pressure control and temperature control i.e. one control valve in condensate side can handle HEX pressure and natural gas temperature ?
Or you need two control valves, one for temperature and the other for pressure control ?

One control valve should be sufficient. If your heat exchanger design is appropriate to allow for controlling heat transfer by controlling condensate level, then you only need a control valve on the condensate. It may be possible to use the gas temperature to directly control the condensate valve, but it would probably be more stable to use cascade control where the gas temperature would control a condensate level control loop.

You would not use level control if there is the possibility of freezing the condensate or causing steam hammer.  

If I understand correctly, you need 2 control valves: one is on the shell inlet line controling the tube outlet gas temperature, and the other one is on the shell condensate outlet controling the level to maintain certain pressure.

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