Tenant did not pay for Gas. NGRID's reaction?

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A tenant whose lease ended and who has moved out did not pay for gas and I am not sure if they had at all registered it to their name as they moved in. The lease does say that they are responsible for paying for it. Gas has never been on my name as it's always been a rental unit and names on service has changed from tenant to tenant. Apparently this last tenant never bothered to switch service to his name since the gas company continued to supply him with free gas.

I am in the Boston area in Massachusetts and NGRID disconnected service for the unit. My new tenant could not be provided with gas or sign up for new service since NGRID is asking for payment...

I'm going to call NGRID but before I do so I wanted to know if ultimately I would be the one responsible for paying the previous bill or I could direct them to the tenant on the lease, which clearly says he's liable for it.

Thanks in advance for your help.