My experience has been to use a central synergy tracker listing all identified and approved synergies part of the deal approval with the acquiring company. This has been both simple solutions in Excel, Power Point or similar tools or more complex software solutions supporting the entire deal flow.
Most importantly is the constant and ongoing focus on the identified synergies and if they are captured. Therefore, this must be a top priority for Deal Team, IMO/PMO team, and the SteerCo responsible for executing the deal and integration.
Including realization of synergies in the ongoing IMO/PMO reporting is key to keep focus on synergies and how to realize them.