If the limiting heat transfer is on the tube side, and if blocking/removing tubes will increase the flow velocity through the remaining tubes (such as with a centrifugal pump), then the impact will be minimal.
I have seen heat exchangers with 15% of the tubes plugged that still are within a few % of the original design heat transfer.
However if the limiting heat transfer is on the shell side, or flow velocity in the remaining tubes will stay the same then you will loose as much heat transfer as you loose in surface area.
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Plymouth Tube