Pulled directly from Brownstoner forum:

I attended a seminar at National Grid’s offices yesterday that detailed some of the rebates and incentives currently being offered.
Of particular interest (to me anyway) is the oil-to-natural gas conversion boiler being given for $699 up to 245,000 BTUs.
A steam boiler of this size normally costs about $2,700 to purchase outright.
Of course, installation and options are priced separately.

If the conversion includes the installation of an indirect water heater, an additional $300 rebate is paid to the customer.

Mid efficiency upgrades:
For customers currently using gas for heating, a boiler upgrade to 85% efficiency or more, up to 300,000 BTUs pays a rebate of $500.
Boilers 300,000-499,000 BTUs at 85% efficiency pay the customer $1,000.

High efficiency upgrades:
Hot water heating system boilers that are upgraded to 90% efficient or greater units, up to 300,000 BTUs pay a $1,000 rebate.
Boilers sized 301,000-499,000 BTUs pay $1,500.
In addition to this, the federal government is offering a 30% rebate on the installation of a boiler of this type with a cap at $1,500.

Indirect-fired water heaters which are rapidly increasing in popularity, installed as part of a conversion OR an existing gas customer’s upgrade pay $300.

Steam boilers rated at 82% efficiency (must have an intermittent ignition system, no pilot light) replaced as a gas-to-gas unit pay $200.

The installation of an outdoor boiler reset control will pay the client $100.

Replacing steam traps in a two-pipe steam heating system will pay $25 per trap.

To qualify for these rebates, it is not necessary to use a National Grid “authorized” contractor. Hiring a plumber or heating contractor licensed by the NYC DOB is all that is required.
Here’s one: