Hello Tabby27 and welcome
If your IVA has finished and you have received your certificate of completion then yes you will be able to keep any PPI compensation you are now offered. Just as an aside, the bank in question cannot even threaten to keep the money to offset the debt that they were not paid because now your IVA is completed they have been required to right off any unpaid debt. As such there is no longer any existing debt and there is nothing to offset against.
To find out more, have a look at this article: Can I claim for PPI mis-selling after my IVA has finished?