Nitrogen is inherently a diluent; so, if you can do so, I'd use CO2 as the fluidising medium, as this will be part of the syngas reaction system (CO2 + C = 2.CO). If you are cleaning the syngas it is also possible to use some of the syngas for fluidisation .... provided it is clean enough.
It appears to be an ill-posed question, Syngas is synthesis gas composed of CO + H2 gas that gets converted to liquid hydrocarbons in Fischer-Tropsch process. If one has already a plastic, depending on the nature of plastic, one can get in thermal process, liquid fuel I am unable to understand why fluidization and then why nirogen. These are my thoughts. I cannot say more.