Sorry for the diversion there - I couldn't help it based on don's rather tongue in cheek comment....
As ever a diagram, picture, diameters of tubes would help us understand - can you get access to the tube plate?
All else fails can you remove the tube sheet easily or is this barely worth cutting it out, cutting the tubes and repairing it?
What have you tried so far?
Could you drill it from the end?
So much depends on access or lack thereof.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.