A computer-implemented method for electronic transaction message conversion to integrate a single-message scheme into a point of sale (POS) device , comprising: receiving an authorization request message from the POS device , the authorization request message comprising transaction data; simulating processing of the authorization request message by an authorization simulation module to: validate the authorization request message; store the transaction data in a transaction data database; generate a provisional authorization response message; send the provisional authorization response message to the POS device ; receiving a capture message from the POS device , the capture message having been generated by the POS device in response to the provisional authorization response message; validating the capture message; translating the capture message to a composite message, the composite message being single-message system (SMS)-compatible; and sending the composite message to a payment network; wherein the authorization request message and the capture message are dual-message system (DMS)-compatible; and wherein translating the capture message to a composite message comprises: converting a first message type indicator of the capture message to a second message type indicator corresponding to the composite message; merging at least a portion of transaction data retrieved during the validation step with transaction clearing and settlement data of the capture message; and inserting the second message type indicator and the merged transaction and transaction clearing and settlement data in the composite message. receiving an authorization request message from the POS device , the authorization request message comprising transaction data; simulating processing of the authorization request message by an authorization simulation module to: validate the authorization request message; store the transaction data in a transaction data database; generate a provisional authorization response message; send the provisional authorization response message to the POS device ; validate the authorization request message; store the transaction data in a transaction data database; generate a provisional authorization response message; send the provisional authorization response message to the POS device ; receiving a capture message from the POS device , the capture message having been generated by the POS device in response to the provisional authorization response message; validating the capture message; translating the capture message to a composite message, the composite message being single-message system (SMS)-compatible; and sending the composite message to a payment network; wherein the authorization request message and the capture message are dual-message system (DMS)-compatible; and wherein translating the capture message to a composite message comprises: converting a first message type indicator of the capture message to a second message type indicator corresponding to the composite message; merging at least a portion of transaction data retrieved during the validation step with transaction clearing and settlement data of the capture message; and inserting the second message type indicator and the merged transaction and transaction clearing and settlement data in the composite message. converting a first message type indicator of the capture message to a second message type indicator corresponding to the composite message; merging at least a portion of transaction data retrieved during the validation step with transaction clearing and settlement data of the capture message; and inserting the second message type indicator and the merged transaction and transaction clearing and settlement data in the composite message. <EOS>