SEZ sales in GST

As per section 16 of IGST Act, following supplies are treated as zero rated supplies:

(a) export of goods or services or both; or
(b) supply of goods or services or both to a Special Economic Zone developer or a Special Economic Zone unit.

Since you are supplying to a SEZ unit, you need not to charge GST.

Input credit on supplies made to SEZ units

(2) Where the goods or services or both are used by the registered person partly for effecting taxable supplies including zero-rated supplies under this Act or under the Integrated Goods and Services Tax Act and partly for effecting exempt supplies under the said Acts, the amount of credit shall be restricted to so much of the input tax as is attributable to the said taxable supplies including zero-rated supplies.

Refund of input credit on supplies made to SEZ Units

(3) A registered person making zero rated supply shall be eligible to claim refund under either of the following options, namely:––

1. he may supply goods or services or both under bond or Letter of Undertaking, subject to such conditions, safeguards and procedure as may be prescribed, without payment of integrated tax and claim refund of unutilised input tax credit; or
2. he may supply goods or services or both, subject to such conditions, safeguards and procedure as may be prescribed, on payment of integrated tax and claim refund of such tax paid on goods or services or both supplied, in accordance with the provisions of section 54 of the Central Goods and Services Tax Act or the rules made thereunder.

So you can claim input credit even if you are not charging tax on supplies made to SEZ Units.

Further you can take refund of supplies made to SEZ units by following above provisions.