We don't know the nature of the fouling. But the options I've seen implemented. None is completely out of problems. You have to evaluate your options with the people involved at the site.
- Steaming: connect a low pressure steam line and circulate thru the fouled section. If is something that melts, dissolves in water, it may help. If it is polymer, It can make your problems even worse.
- Indirect heating: same as before, but steam is on the other side. In both cases you should verify that the exchanger can withstand the temperature. Materials are probably OK, internal tensions are the problem.
- Chemical cleaning. Ideally something that only dissolves the fouling and do no attack the metal. You can combine with indirect heating.
- Brute force: Nobody would recommend this. You can damage equipment badly. But sometimes it works.
- "Cut and Paste". Cut with a non sparking, low heat generation tool. Clean. Re-weld. Depending on the jurisdiction where the plant is located, and the type of equipment, your local pressure vessel authority may allow this modification/repair (or not)
- Keep running with this equipment while you order a new one.